The present invention relates to a trail edge sensor for a printing device and more specifically to a moveable trail edge sensor for duplex registration in a printing device.
In many printing applications, a printing device includes a registration system for determining any skew or misalignment of printable media prior to the print process. Eliminating skew is used to ensure that any images and/or text printed on a printable medium are correctly aligned. Eliminating skew is particularly useful in duplex printing system. A duplex printing system transports a printable medium through a registration system, prints on a first side of the printable medium, inverts the printable medium, transports the inverted medium back through the registration system and runs the medium through the print process a second time. This results in a printable medium with images and/or text printed on both sides. Eliminating skew during the printing of both the first and second sides results in images or text that are properly aligned on both sides of the printable medium. FIG. 1 illustrates a simplified duplex printing system 100.
As shown in FIG. 1, a printable medium is loaded at feeder 102. The printable medium travels down a transport surface to registration device 104. Here, the leading and trailing edges of the printable medium are determined, and any skew is removed via a registration system. Next, the printable medium passes imaging device 106. Imaging device 106 transfer any images and/or text onto the printable medium. After image transfer, the printable medium passes through fuser 108 where the transferred image and/or text is fused to the printable medium. The printable medium then passes to inverter 110 where the printable medium is inverted and passes down along duplex path 112.
The printable medium again is passed to registration device 104. On the second pass, a second side of the printable medium is facing away from the transport surface. Again, the printable medium is aligned to remove any skew, an image and/or text is transferred by imaging device 106, and the transferred image is fused by fuser 108. Having already been inverted and printed on both sides, the printable medium bypasses the inverter 110 and exits the printing system. It should be noted the location of the inverter 110 may be moved to other locations including along duplex path 112. Additionally, it should be noted that in some applications the printable medium may be inverted again after the second side is printed such that the first side is facing up as the printable medium exits the duplex printing system.
FIG. 2a illustrates an exemplary printable media transport system including an exemplary registration device 104. As the printable medium passes through the transport system, it is propelled by transport nips 202 and passes over various sensors. As the printable medium approaches registration device 104, it passes over a first side edge sensor 204a. As the printable medium advances, it passes over a second side edge sensor 204b. These side edge sensors are used to determine any skew in the printable medium. Registration device 104 adjusts the speed of nips 205a and 205b to adjust the skew of the printable medium. The printable medium also passes over a leading edge sensor 206. Sensor 206 measures the arrival time of the lead edge of the printable medium. As the printable medium continues through registration device 104, it passes a second leading edge sensor 208. Sensor 208 measures the arrival time of the lead edge of the printable media and determines and adjusts the velocity of the printable medium by adjusting the speed of transport nips 202. After the velocity is adjusted, the printable medium is passed to registration datum 212 where the registration process is completed and the image is transferred to the printable medium.
The printable medium follows a similar path to that described in FIG. 1, it is inverted and returned to the registration device 104. In this pass, the second side of the printable medium is facing the imaging device, and what was previously the leading edge is now the trailing edge. The trailing edge is measured in the second pass to maintain consistency between the two passes that the printable medium makes through the registration device because it was previously the leading edge of the printable medium before inversion. Again, any skew in the printable medium is adjusted, and the printable medium is passed towards the registration datum 212. In the second pass, however, trail edge sensor 210 is used to measure the trail edge of the printable medium. The velocity of the printable medium is determined and adjusted based upon previous measurements of the leading edge in the first pass, and the printable medium is passed to the image transfer device. It should be noted that a single trail edge measurement is taken in this pass as the printable medium approaches registration datum 212. Multiple trail edge sensors 210 may be included to accommodate different process direction sizes of printable media.
A registration system such as the one illustrated in FIG. 2a has inherent drawbacks though. For example, during the fusing process, heat is applied to the printable medium which may result in a slight deformation of the medium resulting in dimensional changes in the medium. For example, the length of the printable medium may change, which would result in an inaccurate velocity measurement of the medium. Another drawback is the printable medium is passed over additional transport nips after registration. Any variance in speed in the transport nips may result in skewing the printable medium prior to image transfer. To overcome these drawbacks, some printing systems eliminate the intermediate transport nips between registration and passing the medium to the registration datum.
FIG. 2b illustrates an exemplary printable media transport system where the intermediate transport nips have been removed between the registration device 204 and the registration datum 212. The process for printing the first side of the printable medium is similar to that as discussed above in the discussion of FIG. 2a. As the printable medium travels towards registration datum 212, sensors 204a and 204b measure any skew and nips 205a and 205b adjust the lateral position of the printable medium to eliminate the skew. Leading edge sensors 206 and 208 measure the leading edge of the printable medium as it passes to determine the velocity of the medium. The velocity of the printable medium is adjusted and passed to registration datum 212 for image transfer and fusing. The printable medium is then inverted as before.
After inversion, the printable medium is passed through registration device 204 again with the second side of the medium facing up. Similar to the system described in FIG. 2a, the trail edge is now measured by trail edge sensor 210 to determine and adjust velocity prior to printing. However, trail edge sensor 210 is positioned such that the measurement of the trail edge of the printable medium occurs while the registration device 204 is correcting any skew to the printable medium. If there is an adjustment made to correct skew, the measurement made by the trail edge sensor is inaccurate as the position of the trail edge of the printable medium may change based upon the skew correction.
Both of these registration devices include their drawbacks, primarily inaccurate measurements after the paper is inverted and prior to transferring the image to the second side of the printable medium. As discussed above, inaccurate measurements may lead to incorrect printing on the printable medium as the two printed sides may not properly align.